1. Field of The Invention
The present invention relates to a pressure control valve which controls fluid pressure in accordance with electrical signals, in particular, to a pressure control valve preferable to control an automatic transmission for a motor vehicle. Typical types of the pressure control valve are a diaphragm type receiving a feedback pressure on the diaphragm, and a spool type receiving the feedback pressure on the spool end. The present invention relates to the spool type pressure control valve, particularly to the structure of spool.
2. Description of The Prior Art
A spool type pressure control valve, in general, has a pressure modulating section including a spool which has two same diameter lands at both ends and is movable, and a magnetic section having a coil assembly, a core and a rod placed coaxially with, and pressed by a spring toward the modulating section. And a spring is attached to a plunger. The difference between a spring force applied to one end of the spool through the rod, and a force by magnetic power is modulated to balance with a feedback force according to an output pressure. And by the move of the spool, the lands of the spool switches an application port and a drain port made on a valve sleeve. Consequently an application pressure from the application port is modulated and taken out from the output port as the output pressure according to the electric level sent to the magnetic section. (refer to U.S. Pat. No. 4,535,816).
Then, in the case of the pressure control valve having the same diameter lands at both ends, to obtain large output pressure by using the magnetic section including the spring the following equation is applied; EQU P=(Fsp-Fsol)/A (1)
where
P: output pressure PA1 Fsp: spring force PA1 Fsol: force developed by the magnetic section PA1 A: cross section area of the spool end
Due to the above formula, it is understood that to get larger output pressure, only reducing the cross sectional area of the spool is required, but to obtain very large output pressure very small spool is also required. However to attain the above, machining of spool and valve sleeve becomes quite difficult, and furthermore the diameter of the rod shall be also reduced. Consequently the relevant dimensions of the magnetic section have to be revised.
The pressure control valve mentioned above includes the spring to set the range of modulation in the magnetic section, so when assembling, installing the components is not smoothly performed, and coupling of the modulating and magnetic sections is not also performed smoothly.
It is not easy to change the modulation range because the spring has to be exchanged. And in the case that the magnetic section is used with another pressure modulating section utility is quite limited.
In the pressure control valve mentioned above, as shown in the formula (1), the output pressure P is decreased according to an increment of the electric level. If the electrical signal is cut, the maximum output (P=Fsp/A) comes out suddenly, and this sudden high output may damage apparatus equipped in the output side. Considering this upsurge, there is a pressure control valve designed to increase output in accordance with an increment of the electric level. The pressure control valve of this type has merits in view of fail-safe feature, however any changes of properties can not be attained by simple changes of the components.